Melt the chocolate (either a bowl over boiling water, or in small increments in the microwave).
In a saucepan, whisk together the egg yolks, espresso powder, 4 tablespoons of the sugar, and ¾ cup of the cream. Cook over medium heat, stirring constantly with a rubber spatula for 2 minutes, until the coffee dissolves and is blended completely. The color will be a caramel brown.
Add the mixture, to the melted chocolate and whisk until smooth. Refrigerate until completely chilled.
Beat the remaining cream and sugar until stiff.
Remove the chocolate mixture from the fridge, and whisk a little until smooth and mix-able, then fold the whipped cream in.
Divide mousse into serving cups, or bowls, and chill until needed (at least an hour).
Decorate with whatever you like (whipped cream, chocolate covered coffee beans, chocolate shavings... or not at all!